Ph.D. (Doctor of Philosophy)
Department of Physics
Materials science; Condensed matter physics; Manganite; Oxide minerals; Hexagons; Thermogravimetry
This dissertation presents evaluation results of hexagonal Y1-x RxMnO3+delta (R = Er, Y, Dy, Pr, La, Tb and Ho) rare-earth manganites for prospective air separation applications. In these materials, oxygen content is sensitively dependent on the surrounding conditions of temperature and/or oxygen partial pressure, and therefore they exhibit the ability to selectively absorb, store, and release significant amounts of separated oxygen from air. This study presents a full characterization of their thermogravimetric characteristics and air separation capabilities. With the expected potential impact of oxygen content on the physical properties of these materials, the scope of this work is expanded to explore other relevant properties such as magnetic, transport, and dilatometric characteristics.;Single-phase polycrystalline samples of these materials were achieved in the hexagonal P63cm phase through solid state reaction at elevated temperatures. Further annealings under reducing conditions were required for samples with large rare-earth cations in order to suppress the competing perovskite structure and form in the anticipated hexagonal phase.;Thermogravimetric measurements in oxygen atmospheres demonstrated that samples with the larger R ionic radii show rapid and reversible incorporation of significant amounts of excess oxygen (0.41 > delta > 0) at an unusual low temperature range ~190-325 °C. The reversible oxygen storage characteristics of HoMnO3+delta and related materials shown by the fast incorporation and release of interstitial oxygen at easily accessible elevated temperatures of ~300 °C demonstrate the feasibility and potential for low-cost thermal swing adsorption TSA process for oxygen separation and enrichment from air. Neutron and X-ray powder diffraction measurements confirmed the presence of three line compounds RMnO3+delta, the oxygen stoichiometric P6 3cm (delta = 0 for all R), the intermediate oxygen content superstructure phase R3c (delta ~ 0.28 for R = Ho, Dy, Dy0.5Y0.5, and Dy0.3Y0.7) constructed by tripling the c-axis of the original unit cell, and the highly oxygen-loaded Pca21 phase (delta = 0.40 for all R). In-situ synchrotron diffraction showed thermal stability of these single phases and their coexistence ranges, demonstrating that the stability of the delta = 0.28 phase increases with the ionic size of the R ion. The magnetic properties of the multiferroic RMnO3+delta were found to be dependent on the oxygen content of these compounds. Below the magnetic ordering temperatures, samples with higher oxygen content showed slightly decreased magnetization relative to the less oxygenated ones. Dilatometry measurements suggest that the thermal expansion coefficient TEC of the oxygen-loaded Pca21 phase is slightly larger than that of the stoichiometric P63cm phase. The calculated Pca21 to P63cm chemical expansion coefficient 14.38 x 10-3 [mole-O]-1 was found to be within the expected range for the hexagonal Y0.97La0.03MnO3+delta sample.
Abughayada, Castro, "Air separation and oxygen storage properties of hexagonal rare-earth manganites" (2015). Graduate Research Theses & Dissertations. 1047.
Northern Illinois University
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Advisors: Bogdan Dabrowski.||Committee members: David Hedin; Laurence Lurio.